We have proposed to define and compare the subunit properties of avian and mammalian tumor virus RNA- directed DNA polymerases. More specifically, we propose to (a) isolate the DNA polymerase subunits of avian myeloblastosis virus, (b) localize and characterize the DNA polymerase and ribonuclease H activities, and (c) determine by in vitro reconstitution experiments the functions of each subunit polypeptide in RNA-directed DNA synthesis. Furthermore, we are attempting isolation or reconstitution of a DNA polymerase complex from a mammalian RNA tumor virus that has the properties of the two-polypeptide complex of avian myeloblastosis virus, including RNA -directed DNA synthesis. Finally, we will purify and characterize a RNA-directed DNA polymerase of RNA tumor from human Hela cells. BIBLIOGRAPHIC REFERENCES: Bauer, H., Daams, J.H., Watson, K.F., Moelling, K., Schaefer, W. and Gelderblom, H. Oncornavirus-like particles in HeLa cells. II. Immunological Characterization of the virus. Int. J. Cancer, 13, 254-261 (1974). Watson, K.F., Moelling, K., Gelderblom, H. and Bauer, H., Oncornavirus-like particles in HeLa cells. III. Biochemical characterization of the virus. Int. J. Cancer, 13, 262-267 (1974).